Deep Diamonds Shed Light on Ancient Continental Movements
Earth's tectonic plates, immense slabs of rock that bump and scrape past each other to cause earthquakes and trigger volcanism, are some of the world's largest geological structures. But exactly when these plates first began jostling around as they do today has long been a subject of lively debate. Now, scientists who've analyzed some of the planet's tiniest geological structures—the mineral imperfections inside diamonds—contend that modern-style plate tectonics began about 3 billion years ago.
Deep inside Earth, hellish heat and pressure have been cooking rocks and altering their chemical structure since the planet formed about 4.5 billion years ago. But not everything gets roasted. Tiny blebs of mineral that became trapped in diamonds billions of years ago remain pristine, because diamonds are for the most part chemically inert. "In essence, diamonds are time capsules from Earth's mantle," says Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C.
In their new study, Shirey and colleague Stephen Richardson of the University of Cape Town in South Africa combined their own data about the age and chemical composition of diamond inclusions with similar information from other studies published during the past 25 years. In all, the researchers analyzed nearly 4400 inclusions, the vast majority of which were made of silicates, the world's most common minerals.
The duo found that inclusions that formed more than 3 billion years ago fall into a class of minerals known as peridotites, the most abundant type of rock in the mantle, the thick layer of viscous, largely molten rock that lies between Earth's crust and outer core. But inclusions that formed less than 3 billion years ago predominantly fell into a category of minerals known as eclogites, which are thought to form when basaltic minerals, such as those formed at the sea floor at midocean ridges, are carried to great depths and transformed by the heat and pressure there.
The first appearance of eclogitic inclusions approximately 3 billion years ago denotes the origin of modern-style plate tectonics, the researchers contend today in Science. Before then, they say, there's no evidence that crustal plates were being carried from the shallow depths where they formed to great depths. But after that time, the widespread presence of eclogite inclusions hint that great slabs of relatively dense ocean crust were being shoved beneath the edges of lighter continental crust, recycled in Earth just as they are today.
Before 3 billion years ago, convection in the mantle was relatively rapid, so heat was transported from Earth's core rather quickly. In addition to this rapid cooling, tectonic plates, including the light bits of crust that collided to form protocontinents, were small. But after that, mantle convection slowed, Earth cooled less quickly, and the long-term tectonic cycle that forms and tears apart supercontinents such as Pangaea began.
"The team's findings are a quite reasonable interpretation of the data," says Claude Jaupart, a geophysicist at the Paris Geophysical Institute. "This is the first study to see a specific change in inclusions at a specific time in Earth's history."
There's no question that the eclogitic inclusions are bits of basalt that recrystallized deep within Earth, says Robert Stern, a geoscientist at the University of Texas, Dallas. However, he notes, it's possible that some of that eclogite formed on the lower surface of a mass of continental crust and then dripped down into the mantle, a process that some seismic studies suggest is happening in some regions today—an alternative process whose results, billions of years later, would be difficult to distinguish from the effects of plate tectonics. "This is an interesting set of data, but it's not compelling to me."
Nevertheless, Stern adds, "the question of when plate tectonics began, and what was going on before that time, is the number one problem in the solid earth sciences today. ... I commend [Shirey and Richardson] for tackling it."